The present invention relates to a method for determining the rotor position of an electromotor having a plurality of motor phase windings, for example, a brushless DC motor and to a detection module for this purpose.
In many areas of technology, especially in the motor-vehicle area, so-called brushless DC motors have recently been used, which are also known as BLDC motors (BLDC=Brushless Direct Current) having no brushes, which are subject to wear. Instead of a mechanical commutation, in BLDC motors an electronic commutation is provided, which is generally realized through a power electronics. BLDC motors are also called electronically commutated DC motors, or EC motors (EC=electronically commutated).
A BLDC motor is driven by a power-electronic actuator that functions as a commutator, for example, having a 6-pulse bridge converter, which, using pulse modulation, in general, pulse width modulation, produces from a battery or an intermediate-circuit DC-voltage a three-phase AC voltage system that is variable in frequency and voltage amplitude, so that for the BLDC motor current units are made available that are, for example, essentially rectangular. In this context, supplying current to the motor phase windings of the electromotor, i.e., supplying current to its windings, is carried out as a function of the specific rotor position. Usually, two phase windings are supplied with current simultaneously. In this context, the arms of the bridge converter, that are assigned to the phase windings, are active and supply current to a first phase winding in a positive charge and to a second motor phase winding in a negative charge. In this context, the switches of a third bridge converter arm are opened, and therefore the third arm is inactive.
As a result of the rotor of the electromotor, which has, e.g., a permanent magnet arrangement, a countervoltage is induced in the motor windings, i.e., in the specific motor phase windings. For a high degree of motor efficiency, the motor phase windings should be supplied with current such that the highest possible phase countervoltage, having the same polarity as the specific phase current, is induced in them.
In any case, in a BLDC motor, its instantaneous rotor positions must be known for determining the optimal commutation times. The rotor positions, i.e., the commutation times, can be determined, e.g., by a sensor arrangement or also without the use of sensors, for example, by evaluating the zero crossing points of the induced countervoltages in the phase windings that are not supplied with current. From the zero crossing points, it is possible to determine the rotor angle for the next commutation by extrapolation. However, this method is only suitable for electromotors that are run in continuous operation, e.g., in pumps or ventilators.
In motors having an automatic speed control that includes motor standstill, for example, in motors for positioning drives, more dynamic methods are required for determining the rotor position. In this context, certain difficulties undoubtedly arise:
Through the pulse width modulation (PWM) of the switches in the two active arms of the bridge converter, the current flowing in the active, current-supplied motor phase windings is adjusted and limited. Nevertheless, as a result of the pulse width modulation, interference pulses are generated, which are transmitted to the inactive phase winding, that is not supplied with current, inter alia, due to coupling inductances between the individual motor phase windings, so that the zero crossing point of the induced voltage cannot be measured in the inactive phase winding reliably and without distortion. The clock distortions for evaluation must be filtered out of the respective measuring signal.
However, in this context, in analog filters, phase shifts, among other things, occur, which generate the interfering measuring errors.
A digital filtering method is proposed in U.S. Pat. No. 5,859,520. In this method, a zero crossing point of the induced voltages is measured by clocking an upper switch of a bridge arm, so that a freewheeling current is generated via the lower switch of the same bridge arm. In freewheeling operation, the countervoltage that is induced in the motor phase winding that is assigned to the bridge arm in question is measured with respect to a ground potential of a measuring circuit. In this context, it is disadvantageous, on the one hand, that only the upper switches of the bridge arm can be clocked and, on the other hand, that by clocking the upper switches, so to speak, a forced freewheeling operation is generated in order to be able to carry out the measurements at all. However, from the forced freewheeling operation, the result is a reduced potential utilization of the motor.
To determine the rotor position, the polarity of the motor phase voltages induced in the motor phase windings is ascertained. For this purpose, the specific motor phase voltages are compared to reference values, specifically to a real or simulated, virtual star-point voltage, that is applied at a star point, and in each case polarity values are calculated. To avoid distortions that falsify the polarity values, a synchronization to the starting times for supplying current to the motor phase windings is carried out, and a predetermined delay is imposed, in which the polarity values achieve a steady-state, stable condition. Then, the polarity values are ascertained, and, for example, the polarity values are supplied to a control unit for controlling the electromotor.
The method according to one embodiment of the present invention is carried out together with a freewheeling method or methods. The necessary evaluation circuit may be compact and intergrated, for example, in a circuit for controlling the electromotor and/or for controlling the power electronics that supplies power to the electromotor. The method according to an embodiment of the present invention is applied over a large speed range in a variable manner, and no measuring errors arise, such as are caused by phase shifts, in the case of methods using analog filters.
In one embodiment of the invention, the polarity values are ascertained and read out only if no freewheeling of the electromotor has occurred during the delay, avoiding interference in the polarity values.
After the preestablished delay has elapsed, a sampling period commences, in which for every motor phase winding not only a single polarity value, but also, in accordance with the length of the sampling period, a plurality of polarity values are determined. The polarity values are each stored in a storage unit. For example, a polarity value that was calculated later may replace a polarity value that was calculated earlier. In one embodiment, only the most recent polarity value of a motor phase winding is stored, and the most recently determined polarity value is then read out.
The sampling period can be terminated as a result of a plurality of events, e.g., as a result of a freewheeling of the electromotor or as a result of a subsequent switch-on time point for supplying current to the electromotor.
As was mentioned above, in conventional methods, the electromotor is supplied with current using pulse width modulation for setting and limiting the currents flowing in the individual motor phase windings. The switch-on times for supplying current to the motor phase windings of the electromotor, in this context, are preferably defined by a pulse-width-modulation basic timing signal. This pulse-width-modulation basic timing signal may be used for synchronization in determining the polarity values.
It is also possible that the pulse-width-modulation basic timing signal is started anew, in each case, by a synchronization signal in response to a commutation of supplying the electromotor with current. In a commutation, the current supply generally alternates from one pair of phase windings to an adjacent pair of phase windings. In one alternative embodiment of the present invention, the determination of the polarity values is synchronized anew, in each case, on the basis of the synchronization signal. Preferably, both the synchronization signal as well as the pulse-width-modulation basic timing signal help in synchronizing the measuring of the polarity values. For example, by combining the basic timing signal and the synchronization signal in a logical xe2x80x9cOR,xe2x80x9d the timing of the measurement of the polarity values may be realized.
The motor phase voltages and the star-point voltage, may be measured at the respective motor phase windings, or at the star point of the motor phase windings in alternative embodiments. However, these actual measuring points are often inaccessible. Therefore, the motor phase voltages and/or the star point voltages are simulated in one preferred embodiment of the invention.